Recent electronic equipment such as a digital camera or a mobile phone having a camera function includes a strobe apparatus having a rod-like flashtube, for example.
The flashtube of the strobe apparatus essentially includes the following elements:                a glass tube;        an anode electrode as an anode-side electrode disposed at one end of the glass tube; and        a cathode electrode as a cathode-side electrode disposed at the other end of the glass tube (for example, Patent Literature 1).Generally, the glass tube of the flashtube is made of borosilicate glass, and the anode electrode and cathode electrode are made of tungsten in consideration of durability against thermal shock in light emission, for example.        
Digitization of picked up data has enabled failed picked-up data (e.g. photograph) to be erased, so that the number of photographs taken with electronic equipment including a strobe apparatus has recently increased significantly. The number of light emissions by the strobe apparatus also has increased significantly. Therefore, the strobe apparatus is required to have higher durability for light emission (emission durability) than ever.
When a crack appears in the glass tube of the flashtube due to stress during flash light emission, however, rare gas filled in the flashtube leaks or the strength of the glass tube of the flashtube reduces, thereby reducing the durability of the strobe apparatus. Therefore, it is required to increase the strength of the glass tube of the flashtube.
It is conventionally known that, in order to increase the strength of the glass tube of the flashtube, the glass tube is made of quartz glass. Generally, quartz glass is mainly made of silicon dioxide, and has a characteristic where the melting point is about 2000° C. and the thermal expansion coefficient is about 0.55×10−6/° C. The quartz glass is resistant to thermal shock occurring during light emission of the flashtube, and has high transmission in the light wavelength region from ultraviolet to infrared. The quartz glass having high strength and high transmission is in widespread use for a glass tube of a flashtube.
Tungsten used in the anode electrode and cathode electrode of a flashtube has a melting point of about 3400° C. and a thermal expansion coefficient of about 4.5×106/° C., and hence has sufficient durability against thermal shock.
However, the melting point and thermal expansion coefficient of quartz glass are significantly different from those of tungsten. When the anode electrode and cathode electrode are directly bonded to the glass tube by heating and melting the glass tube, a crack appears in the glass tube or electrode sealing section due to difference in melting point and thermal expansion coefficient, disadvantageously.
In order to address this problem, the following method is used:                a bead section that is formed of several kinds of glasses and has a thermal expansion coefficient between the thermal expansion coefficient of quartz glass and that of tungsten is welded and fixed to the glass tube; and        the tungsten used as the anode electrode and cathode electrode is fixed to the glass tube via the bead section fixed to the glass tube.        
As another method, borosilicate glass is coupled to quartz glass via stage joint glass formed of several kinds of glasses having different thermal expansion coefficients, for example, thereby increasing the strength of the glass tube of the flashtube. In other words, the stage joint glass absorbs the difference in thermal expansion coefficient between the quartz glass and the tungsten used as the anode electrode and cathode electrode.
When the strength of the glass tube is increased, the durability of the glass tube is increased. However, the durability of the anode electrode and cathode electrode is the same as ever and hence the durability of the whole flashtube does not vary. Therefore, for example, the influence of discharge on the cathode electrode is reduced using a sintered metal body. In this case, the anode electrode is made of tungsten as it is, so that the durability of the anode electrode is lower than that of the glass tube. As a result, the emission durability of the flashtube is low.
Thus, it is required to consider the influence of the emission durability on the flashtube, especially on the anode electrode.
In other words, it is desired that the emission durability of the flashtube is increased by increasing the strength of the glass tube and the durability of the anode-side electrode.